Hi GAinsworth,
As a non-core member but applicaiton developer I thought that I would add a few comments. I work on mission critical applications on a regular basis, in a multi-developer environment, along with near and off-shore resources. We have strict processes in place from DB naming conventions, to build and deployment strategies, yet we manage to have issues as well - some rather large.
In your first post you made the comment " looking at DNN from a business point of view in three critical areas (Repeatable installation, support for current installation, upgrade installation) DNN meets none of them with respect to the attributes of a "Business Critical" implementation.", these are the areas I would personally say DNN excels in. Although one can use fancy tools such as FinalBuilder, Nant, Wise, CruiseControl, etc. to maintain a clean environment and upgrade as needed, the core team has very advanced update routines in place to update the database schema, etc. and ensuring that DNN is easy to maintain and update. You would see this, for example, when upgrading from DNN 3.11 to DNN3.2. In almost every case all I need to do is extract the update zip file to the web directory, open up IE and the update process kicks off! The documents that come with the project, especially the installation document gives step-by-step instructions for how to update, and notes critical steps such as backing up your web.config file, etc. Also, Microsoft provides support such as the Membership/Roles API that wasn't yet released to the public (some may argue this is taking a chance, too, since it is new...). I would encourage you to take another look at this and look to network with others. There are a couple of books on this portal in press that are also very helpful.
If you do commit to using this portal you are, of course, committing to its framework. The importance of this is that should you decide to extend or create your own custom modules you will need to code within the established framework or you run the risk of your code forking, which would make it difficult to maintain when updates come along. There really isn't any reason to deviate from the framework and it is very solid. I'm not saying it is easy - imagine the amount of hours and number of bright people that have been working on this. The support from a user community working on a free open source portal is fantastic! The amount of time and even free service people have provided make you really feel a part of this project, and you always have the opportunity to contribute.
I do not dismiss your concerns, since I can see how this conclusion can be made. I wish you the best in whatever future decisions you make, and believe that investing in DNN is a very good one!
John